The present invention relates to a method of preparing an .alpha.-sulfofatty acid ester useful as the intermediate of active ingredient for detergent, and it relates particularly to a method of directly obtaining an .alpha.-sulfofatty acid ester by making a fatty acid react with SO.sub.3 in the presence of an alcohol.
The art of preparing an .alpha.-sulfofatty acid ester by sulfonating a fatty acid ester by means of sulfur trioxide is well known. However, this conventional method is more or less accompanied with the hydrolysis of the ester at the time of sulfonation and besides the speed of sulfonation reaction therein is low, so that it necessitates not only the application of excess sulfur trioxide but also the aging for a substantial time subsequent to the sulfonation.
As an alternative to the foregoing method, there is also known a method of preparing an .alpha.-sulfofatty acid ester which comprises sulfonating a fatty acid in the first place and then esterifying the sulfonation product. This method admittedly has an advantage that the speed of sulfonation reaction therein is high, but it is defective in that the viscosity of the reaction product increases remarkably with the progress of said sulfonation reaction.
By the way, an .alpha.-sulfofatty acid ester becomes an active ingredient for detergent subject to neutralization of the sulfonic acid group thereof to turn into an alkali salt. Meanwhile, an .alpha.-sulfofatty acid prepared by either one of the above two methods is colored with the by-products arising from the sulfonation reaction, and accordingly, it is normally required to subject said .alpha.-sulfofatty acid ester to the bleaching treatment prior to subjecting it to the neutralization treatment. This bleaching treatment is generally performed by the use of hydrogen peroxide, and therefore, there is a fear of a part of the ester linkage being hydrolyzed at the time of said treatment. This fear is aggravated when the degree of coloring of .alpha.-sulfofatty acid ester is remarkable as it necessitates application of more severe conditions in performing the bleaching treatment. An hydrolysis of the ester at the time of the bleaching treatment is undesirable for the reason that it will increase the by-product dialkali metal salt of .alpha.-sulfofatty acid, which is inferior in water-solubility and detergency, at the time of neutralization treatment subsequent thereto.
Said dialkali metal salt of .alpha.-sulfofatty acid has in fact no serious influence on the water-solubility and detergency as a whole when the amount thereof is little relative to the amount of alkali metal salt of .alpha.-sulfofatty acid ester. In this sense, the method disclosed in DT-OS 1418887, which aims to increase the yield of active ingredient for detergent through the procedure comprising subjecting a fatty acid ester to sulfonation reaction, subsequently sulfonating an aliphatic higher alcohol with excess SO.sub.3 present in the reaction product and neutralizing thereafter, is admittedly an effective means for relatively reducing the influence of dialkali metal salt of .alpha.-sulfofatty acid which is undesirable as the active ingredient for detergent.
Accordingly, it is a must to obtain an .alpha.-sulfofatty acid ester useful as the intermediate of active ingredient for detergent at the possibly higher yield and in the state of being minimized in coloring for relatively reducing the ratio of .alpha.-sulfofatty acid to be inevitably brought about as a by-product in the subsequent bleaching process to .alpha.-sulfofatty acid ester.